Various embodiments described herein relate generally to particle accelerators, and more particularly to particle accelerators having extraction foils for stripping electrons from charged particles.
Particle accelerators, such as cyclotrons, may have various industrial, medical, and research applications. For example, particle accelerators may be used to produce radioisotopes (also called radionuclides), which have uses in medical therapy, imaging, and research, as well as other applications that are not medically related. Systems that produce radioisotopes typically include a cyclotron that has a magnet yoke surrounding an acceleration chamber. The cyclotron may include opposing pole tops that are spaced apart from each other. Electrical and magnetic fields may be generated within the acceleration chamber to accelerate and guide charged particles along a spiral-like orbit between the poles. To produce the radioisotopes, the cyclotron forms a particle beam of the charged particles and directs the particle beam out of the acceleration chamber and toward a target system having a target material. The particle beam is incident upon the target material thereby generating radioisotopes.
Known cyclotrons direct the charged particles so that the charged particles are incident upon an extraction foil. For example, the extraction foil may be positioned at an outer edge of the spiral-like orbit so that the charged particles reach a predetermined speed prior to being incident upon the extraction foil. When the charged particles hit the extraction foil, the foil strips electrons from the charged particles causing the particles to change polarity and thereby project out of the acceleration chamber.
In conventional cyclotrons that use extraction foils, the foils are held by a frame within the path of the charged particles. At least two edges of the extraction foil may be secured to the frame (e.g., through clamping or the like) such that the edges have fixed positions with respect to the frame. Another edge of the extraction foil may be exposed and positioned within a path of the charged particles. When the charge particles are incident upon the extraction foil, the extraction foil experiences a significant increase in temperature, such as 750 K or more. The significant temperature change causes the foil to change in size (e.g., expand). The size change is based on the material of the foil and the coefficient of thermal expansion of the material.
Such extraction foils are susceptible to failure. The portions of the extraction foil that are secured by the frame may experience stresses caused by the clamping forces of the frame. In addition, the portion of the extraction foil that receives the charged particles experiences a very significant temperature change. Moreover, the change in size caused by the temperature change creates additional stresses on the extraction foil because the frame holds the edges in fixed positions. More specifically, when the edges have fixed positions, the extraction foil is incapable of expanding or contracting within a plane. Instead, portions of the extraction foil may buckle and/or stretch. Accordingly, the above stresses may cause damage to the extraction foil that eventually leads to foil failure. Although damaged extraction foils may be replaced, such procedures have undesirable consequences. First, the procedure for replacing extraction foils increases radiation exposure to personnel. Second, during the replacement procedure, the cyclotron is not in operation.
Accordingly, there is a need for a particle accelerator that increases the lifetime operation of the extraction foils thereby reducing the frequency of foil replacement.